


Putting the "Blew" in "Blueberry"

by spaceyacey



Category: Undertale (Video Game)
Genre: Alternate Universe - Underswap, F/M, First Time, Oral Sex, Plot What Plot/Porn Without Plot, Smut, Some Fluffiness, Underswap Sans, ecto-dick ftw
Language: English
Status: Completed
Published: 2016-02-14
Updated: 2016-02-14
Packaged: 2018-05-20 09:38:23
Rating: Explicit
Warnings: No Archive Warnings Apply
Chapters: 1
Words: 4,368
Publisher: archiveofourown.org
Story URL: https://archiveofourown.org/works/6001153
Author URL: https://archiveofourown.org/users/spaceyacey/pseuds/spaceyacey
Summary: <blockquote class="userstuff">
              <p>You're dating one precious little blueberry muffin and you couldn't be happier. Well, there's one way you could be happier, you suppose. You can't deny it's crossed your mind before, but when Sans starts displaying his own interest in the idea, you decide it's time to jump on the opportunity. </p><p>Underswap!Sans/Reader, pure smutfic. Please enjoy this sinner's offering and dreadful titling skills.</p>
            </blockquote>





	Putting the "Blew" in "Blueberry"

**Author's Note:**

> I was upset by the lack of Underswap!Sans fic on here so I'm writing my first fanfiction in like three years or something. Someone has to deflower that blueberry. So, happy Valentine's Day, sinners.
> 
> (Disclaimer: Fic unrelated to Valentine's Day - it's just mindless smut. Timing is coincidental.)
> 
> Might make some more of these with some general continuity, if you guys like it enough. Enjoy.  
> my tumblr: http://sspaceyacey.tumblr.com/

Dating Sans was the best choice you might have made in your life, ever. The boy is so sweet it makes your teeth ache, enthusiastic in nearly every activity that filled his day, full of the kind of energy you could only sit back and marvel at. Though a touch naïve, childish at times, he’s intelligent, and imaginative and expressive and you can spend _hours_ talking to him about anything. Mexican food. Motorcycles. Knitting. Cooking competition shows. Sans rarely runs out of things to say, and when he does, the two of you just cuddle with each other, content to do nothing more than appreciate each other’s company.

And yeah, he happens to be a skeleton. Friends were usually surprised when you introduced them to him. You’d been surprised when you’d been introduced to him, too, but that was a story for another time. If they had an issue with it, they weren’t your friends anymore. That might seem like an overreaction to some, but most other people didn’t have to deal with the absolutely crushed expression that overtook his sweet skeletal face when he had to endure rejection. You’d never felt such a protective flare in your chest as you did the first time you laid eyes on that expression.

You are so glad you’d met Sans. He’s quickly becoming the light of your life.

It’s an average Tuesday for the two of you – you’ve just gotten off work and stopped by to visit him. As per usual he greets you with an affectionate embrace. Immediately he takes your hand and leads you inside. He chatters at you about some video he saw online, something funny that made him think of you. But you’re a lot more interested in watching him than anything the internet has to offer. You wait until he brings you into the room and the door shuts behind you before making a move. Papyrus isn’t home to walk in on you two, but it’s a habit nonetheless. At first it seemed that you were courting danger in courting Sans, considering the subtle threats his brother had laced into his conversations with you, but he’d warmed up to you before long.

Still, you’d rather not press your luck by getting caught defiling his brother or anything like that. Not that that’s _really_ what you’re doing. You’ve pulled him into a hug, nuzzling your face up against his jaw. “I thought about you all day today.”

“Really?” Sans still sounds so surprised when you say things like that. You’d thought at first that it was just humility that made him behave this way, but you realized after only a couple of weeks of dating that he was genuinely self-conscious. Wherever he got this inferiority complex from, you can’t guess, but all you can do to combat it is to reassure him how special he is at every chance you get.

“You bet. The day went so _slowly_.” Between peppering the side of his face with light kisses, you relay to him all the frustrating things that happened, all the little jokes you wished you could have relayed to him, your snide commentary on dumb remarks your coworkers made throughout the day. His responses vary from hums of sympathy, to bright giggling, to soft sighs, the lattermost becoming more and more frequent as your lips trail lower on his face. “Mostly,” you say, wrapping up the day’s tirade, “I just thought about how bad I wanted to smooch my cute boyfriend.”

His cheeks light up with the soft, magical blue glow of his blush, and though he ducks his head you can tell he’s pleased with the compliment. “I thought about you all day, too,” he says, his eyes closing as you press your lips to the vertebra sticking out from his collar.

“Really? About smooching me?” you ask him teasingly, and he laughs. His eyes pop back open to give you a mock glare.

“Yeah, maybe!” His defensive tone is a joke, you’ve known him long enough to be able to tell. He’s not so immature that he won’t acknowledge the fact that you guys kiss – though he doesn’t like talking about it around his brother. To be fair, neither do you. “If you’re allowed to, so am I. I can daydream all I want about kissing you.” His eyes are sliding shut again, focused on your mouth at his vertebrae. “About how…hmm…how soft your lips are.” You tug on his collar to pull it aside a bit more, revealing his clavicle to you. You press your lips to the bone and let your tongue slide out to prod it. He shivers a bit. “Ah…and your tongue, that too.”

Sans hums with pleasure, and it’s then that you notice that the room…just got brighter? You stop, curious, and your eyes trace the source of illumination to…the front of Sans’ pants. Your eyes widen a bit, and you give him a gentle nudge with your nose to stir him to attentiveness.

"Sans, are you popping a _boner_ for me?" you snicker in the general area where his ear would be, confident that your stupid pun will have him giggling out that cute laugh of his.

Instead he makes a choking sound and pushes you back, eyes darting down to the subject in question. Sans backs away from you until his back hits the wall. You give a start and step closer again, worried. It’s such a contrast to how relaxed he was in your arms.

"Oh, gosh, I - I'm sorry, jeez," he flusters, tugging the front of his shirt down over the front of his pants, attempting to hide the blue glow. It helps very little. His expression is mortified. "That's - I didn't mean for it to - y-you're just so pretty and...and..."

He's really working himself up, and he's so embarrassed you can see bright blue tears forming on the edge of his eyesockets. Your heart jolts at the sight. It's not at all how you want him to look when he's around you, but you should have realized this was how he’d react. He can be so damned sensitive, even when he has no reason to be. Of course this would spook him. The two of you have never been intimate before. You reach out and touch his cheekbone, and he squeezes his eyes shut, still mumbling apologies.

"Sans, come on, don't be like that." Your voice is calm and soothing, and your thumb swipes over his face to catch a tear just as it falls. "There's nothing to be ashamed about, okay?" His only response is a soft whimper, and you sigh as you pull him into a hug. You press the side of your face against his, and soothingly rub at his spine through the back of his shirt.

"I'm glad I make you feel this way." The reassurance in your tone seems to comfort him a bit, as he finally releases the hem of his shirt to return your embrace. You bite your lip, contemplative, wondering how to proceed. This is such new territory for the both of you. The last thing you want is to make him uncomfortable. You turn your head so your lips brush against his jawbone. "If you want, Sans...I could make you feel even better," you murmur, and he freezes, body tense in your arms. Uh oh. Did you take it too far? You softly say his name again, growing concerned. When he doesn't respond, you start to ease yourself back, ready to backpedal. His arms tighten before you can, clutching you close.

"Do you...want to?" Sans asks, hoarsely. Relieved that he's found his voice, you let out a chuckle. That probably wasn't the right choice - he self-consciously hitches his shoulders. Berating yourself inwardly for your insensitivity, you twist in his embrace so you can look in his face. His eyes are still closed, though no tears are gathered around them. You pull one arm free from the embrace and tip his face toward you.

"Hey." At the prompting of that single word, Sans opens his eyes slowly. The blue lights burn so bright and beautiful in his sockets. You smile gently, feeling a swell of affection at the sight. "I really, really do, Sans." Before he can respond, you let the hand at his back trail lower, graze forward, teasing with the hem he'd been holding onto himself only moments ago. Keeping one hand on his face and holding eye contact, you slide the other hand up the front of his shirt and let your fingertips brush his lowermost rib. His teeth clench and a hiss of air escapes between them. "But if you don't like something I do, or if you get uncomfortable and want me to stop, just tell me. Got it?"

You search his eyes for comprehension, and he nods slowly. With this encouragement you continue. Both of your hands now move under his shirt to play with his ribs. It's somewhat familiar territory. The two of you have gotten as far as a little light petting before tonight, and you know this is something he likes. Maybe if you start with something tamer, familiar, he'll loosen up a bit.

It does seem to do the trick, actually. His posture isn't quite so stiff now, and the bright blush that nearly spread all the way down his cervical vertebrae has settled back onto his cheeks. You take this as a sign to proceed. Your lips press against his mouth, and you part them slightly to probe your tongue against his teeth. Sans understands what you want, and he opens his mouth to give you access. Your tongue runs along the inside of his teeth and you're rewarded with a soft, breathy sound before he sends his own tongue out to meet yours.

Wasting no time, you draw the magical appendage into your mouth and begin to suck on it, occasionally letting your teeth drag over it. You don't think you'll ever quite get over the novelty of how it feels. Smoother, slicker and wetter than a human tongue, you can't help but get turned on every single time his tongue meets your own. Sans fares no better on that front; due the kiss and your continued teasing at his ribcage, he's started to softly pant and tremble. It's time to take it further yet again, you think.

You give his ribs one final, harder rake with your fingernails, eliciting a particularly sharp gasp from Sans, before you slip your fingers an inch or so beneath the waistband of his pants. His tongue twitches strongly inside your mouth, and his eyes stretch wide before he snaps them shut again. You let his tongue fall from your mouth, and it hangs from the corner of his now instead. The expression on his face is so fucking hot - desperate, heated, and still just a bit embarrassed. You feel your own arousal flaring, and take a moment to drink in that look, committing it to memory. That'll fuel your fantasies for weeks to come. You're back on track without missing a beat.

"Sans," you murmur as you shift your hands to grip either side of his pelvis. In response he tips his head back, his spine arching toward you. You smile and give his hipbones a firm squeeze, pulling a strangled sound from his nonexistent throat. " _Saaaaans_. Sweetheart. I want you to look at me." He does so, reluctantly, eyes half-lidded and hazy. You press a tender, comforting kiss to his forehead. "You look so gorgeous right now," you tell him.

All he can reply with are a few disjointed stutters, but you don't need to hear him try to brush off your compliments right now. Without warning you pop open the button on the front of his pants and all the words seem to catch in his teeth. A grin, slightly impish, takes hold of your mouth, and you keep your eyes fixed on his as you slide the zipper down, bit by painstaking bit. His eyes widen deliciously with every passing second, and when you hit the bottom of the zipper he suddenly explodes with the words that wouldn't come before.

"Wait! Wait, I-I've never done this with a h - well, I've never done this with anyone, actually, but, my point is, I don't know if...it's just that you're h-human and it might be...weird to you!"

You could have guessed from his behavior that he’s a virgin, but the uncertainty that came with you being a human…that hadn’t even occurred to you. Honestly, you wish you could say you were surprised at that particular concern of his, since you’ve made things work just fine despite the species gap, but you really aren’t.

That self-doubt might be irritating on anyone else, but with Sans it was so endearing that it makes your chest ache. You wanted him to understand that _weird_ didn't matter to you. That the new and unexpected things that you learned about his body only made you care for him more. That it wasn't going to scare you away. You lean in close to him again, pressing against him so he's pushed up against the wall.

"Whatever you have between your legs here, sweetheart," you begin, lightly palming the mass in his underwear. He moans, softly, not enough to drown out your words. "I promise you, I'm going to have a lot of fun playing with it." You rub him through the fabric of his underwear and earn yet another moan. "Especially while you're making such sexy noises." Sans squirms, sheepishly murmuring your name, and you kiss him on the cheekbone. "Please stop worrying, Sans. Just let me take care of you."

Satisfied that you've put him at least a bit more at ease, you drop down onto your knees in front of him. His head drops down to follow you, teeth clenched together nervously. You shine a smile up at him, and he reflects it shakily. God, he's so cute. You shouldn't want to fuck him so senseless, not when he's so sweet and gentle and innocent, but holy hell was there something appealing about the idea of him coming undone at your hand.

Speaking of, you'd better get on that, before he starts to work himself into a fit of nerves again. With his pants unzipped like this, you can make out the shape of the bulge in his pants. Its outline shines slightly through the fabric of his underwear, and it looks... _well, looks phallic enough to me_ , you think, hiding a grin from him. It can't be as weird as he's worried it is, you're sure. After all, considering that lovely tongue of his, you're fairly well acquainted with magical appendages.

Without further ado, you slide his pants and underwear down, letting them catch at his patellae. His response is not unexpected, as he mumbles something under his breath and braces his hands on the wall behind him. His dick is...yeah, normal territory, relatively speaking. It by no means looks like a human penis. Firstly, it's blue, and glowing, though the shape is exactly what one would expect. But it's also much...smoother, lacking in the veins and ridges you'd normally see. Funnily enough, it looks a bit like a sex toy.

"Is it...okay?" You glance up and see Sans staring at you, and realize you've just been intently studying his dick. His whole face is blue again.

"Oh yeah," you say, smile turning wolfish. You take his shiny blue cock in your hand and massage your fingers over the shaft. Its length is fairly average, you note, but damn if it isn't thick. Your fingers can barely meet around it. The oddest thing about it is that it isn’t as warm as you were expecting. Though it isn’t cold, really, it’s definitely cooler than your skin. The difference in temperature is a pleasant sensation, and it must be for him, too, because he lets out the sweetest, neediest little whine.

You start to play with him, testing and teasing, figuring out what he likes. From the sounds he makes…it seems that he likes just about everything you’re doing. It’s not really clear whether or not Sans is trying to form coherent words, but if he is, he’s failing. With every slow, gentle stroke you add another unique noise to the collection. Groans, whimpers, pants, gasps—you relish each one that falls from his lips. You want to hear every possible sound this skeleton can make.

After working him over with your hands for a bit, encouraged by an especially sharp moan, you let your fingers fall from his dick, resting your hands on his femurs. “Keep going, please,” Sans splutters, surprising you. You didn’t think he was still capable of putting three words together right now, and you _really_ didn’t expect him to start begging you to continue. Much as he was clearly enjoying this, you figured his self-consciousness would prevent him from saying such a thing. But he did, and you sigh, smiling patiently. As if you were going to _stop_. Silly little skeleton. In reply you swirl your tongue around the tip of his cock. He sighs out a soft _yesss_ at you, and you resume your explorations, this time with your tongue.

You don’t quite take your time as much with this. It’s clear that he’s getting desperate for release. The heave of his chest is quick and shallow, his phalanges clenching and unclenching unconsciously at the wall behind him. His eyes are shut tight, and it’s clear that he’s totally lost. You’ll take care of him soon—you just want to learn his taste a little. And no, his dick does not taste like blueberries. Like his tongue, it doesn’t taste much like anything, only stimulating a mild tingling sensation in your mouth. That’s just the taste of his magic, you presume. With no further ado, you spread open your lips and take him into your mouth.

The thickness of his cock catches you off guard—it’s really quite impressive, and you really have to work to shift further and further down his shaft. Your slow pace doesn’t bother Sans in the slightest. He’s started to tremble now, the strange rhythm of it turning his constant groans into a hitching sound that seems almost like laughter. Finally, you’re able to slide the whole of his cock into your mouth with no trouble, and when you bury him to the hilt you pause to circle your tongue around his shaft. He yelps at this, and you notice one of his hands come off the wall, hanging in the space above your head. You remove one hand from his femurs, and reach up to twine your fingers between his phalanges. He whimpers appreciatively, and holds tight to your hand as you start to blow him proper.

Your mouth begins to ache after a bit, but you can tell from the increasing pressure on your fingers, from the growing desperation in his noises and the tension in his expression, that it won’t be long now.

"Ah...ahhh...wait!" Sans cries out suddenly, swallowing back a moan. "St-Stop, stop," he begins to plead, and you slide off of his cock with a wet sound. Your brow creases with worry as you look up at him, wondering what's the matter. "I just - uh." His hand trembles as much as his voice as he pulls it from your grasp and brings it up to his sweat-slicked forehead. His eyes dart to your face, and then off to the side. "I was really. Close."

“To…coming?” He nods tersely. You blink. Not really why you were expecting him to stop you. "That's...kind of the point of blowing you," you tell him, unable to stop yourself from chuckling. He petulantly whines your name and, in a fit of embarrassment at your bluntness, tugs the front of his cape up over the ridge of his nasal bone. It’s a childish, silly gesture, and despite the circumstances, your heart warms at how _adorable_ he is.

"I know that!" he exclaims defensively, voice muffled through the fabric. His blush is so bright it glows through the fabric. "Just...you know. Your m-mouth was still..."

Comprehension dawns and you let out a soft _ahhh_ of realization. It’s sweet that he’s concerned. But entirely unnecessary. "That's okay, Sans." You part your lips over the head of his dick again, and he keens at you, trying to disappear further into his cape. Pulling back a bit, you lap your tongue over the head of his cock, relishing his cute little mewls. "I want you to come in my mouth," you murmur, lips brushing the tip every time they come together to form a consonant. Before he can pull his cape entirely over his head and attempt to collapse in on himself like a blueberry supernova, you shove his length back into your mouth, now working him, hard and fast, with both your mouth and hand.

Sans _screams_ your name this time and holy shit you're glad it's just the two of you in the house, because he gets loud, _fast_. The sudden change in gears would have startled you if it wasn't so hot. "A-AH! THA-AAT'S IT! OHHH, THAT FEELS SO - I-I CA-AHHHHHNN! PLEASE, PLEASE DON'T STO-OOOP!" His broken babbling continues, pleas for more mixed with praise and wails and fragments of pure and utter nonsense. You’ve never heard _anyone_ make noises like this, and the fact that your sweet, precious Sans is getting so lost in lust – because of you—it sends a jolt of heat straight between your legs. You’re tempted to touch yourself, but you refrain. This encounter, you decided, is entirely about him. His hips have started to twitch and buck beneath you, and you steady your free hand against the ridge of his pelvis to still him, so he doesn't screw up your rhythm.

He’s so close now, and he doubles over a bit, hands finding purchase on your shoulders. He calls your name again, and you squeeze his pelvis before giving a long, low hum in response. This does the trick, sending him over the edge. Sans comes, with a cry so loud you’d worry for the state of his vocal cords, if he had any. The magical liquid that rushes into your mouth is as cool and tingly as his cock, and you don’t really consider the implications of swallowing it. You just do it. Your hand slows and you draw your mouth up his shaft, resting around the head as you work him through his orgasm. His yelps grow steadily quieter, and his eyes are hazy and unfocused when he looks down at you. But there’s an intensity there, in the gape of his mouth and crease of his brow, that shocks you. You sit back on your heels, letting his dick fall from your lips and his hands slide from your shoulder.

You reach up and wipe away a bit of the fluid that spilled from the corner of your mouth, and hum as you spread it curiously over your fingers. There’s a luminescence to it, as with the rest of his magic. “Funny, I was expecting it to taste like blueberry jam,” you joke, watching as it dissipates entirely, and his dick along with it. Sans starts to laugh, breathlessly, and you laugh with him.

To your surprise he wobbles where he stands, and before you can get up to brace him he drops onto his knees and slumps against you. His bones are rattling, and you hold him close, letting his head rest on your shoulder. You're a little worried by the state he's in, and you stroke his shoulders. It's your turn to be uncertain. "Was that good, sweetheart?" you ask, scared you pushed him too far, too fast.

"Mhmm," he hums into your neck, and you shudder at the vibration of his voice. "S'mazing," he mumbles, half-coherent, and you recognize his tone. He's wearing down now, unsurprisingly, as he always does after overexerting himself. It's how he operates - goes and goes until he crashes. And you certainly _did_ have him going.

"I'm really glad," you say, leaning away from him. He slouches forward but you catch him with a hand on his chest, giving him a gentle push so he rests against the wall. With some effort you manage to pull his underwear back up, followed by his pants. "There are so many more things I want to show you," you tell him quietly as you zip him back up and refasten the button. "There are so many things I want to do to you." You sit back again to look at his face, which is dazed and contented and a little lovestruck. You affectionately brush your hand over his cheek and even in the state he's in, he raises his own to meet it.

"God, you don't even know what you do to me, Sans," you whisper. The words are wasted on him at this point, but you don’t care. He’s spent, he’s not listening, but you still murmur praises to him. You tell him how sweet he is, how sexy he is. You tell him how gorgeous he looks when he comes. You tell him how lucky you are to have met him. You tell him he means the whole world to you.

When you’re satisfied, you peck him between the eyesockets, on the pointed crest of his nasal bone. Despite his obvious exhaustion, he still grins. "Let's get you to bed."

It takes a little work to haul him up off the floor and into his bed, but you manage. Sans curls around you and you know you won't be free until he sleeps off his orgasm. You're fine with that. You'd like a nap yourself. But you'd be lying if you said you weren't eager to get him so wild again. Maybe next time you can actually take that sweet ghost dick of his for a ride.

But for now, you're just happy that he's happy.


End file.
